I've changed the way PCI memory mapping is done, so that it uses the
physical address mappings set up by the BIOS.  This avoids some
potential conflicts if we tried to map over the same region that the
BIOS had used.

The PCI spec indicates that the ROM should have set up the mappings
for all of the devices in a reasonable way, but we all know that specs
don't mean much in the PC world.  If you experience any problems with
the new code (or if it fixes an old problem!), please let me know.